This invention relates to systems for the down conversion of analog signals for generating digital I and Q signals in multiple channels which are aligned in amplitude and in time.
Modern radar systems use monopulse techniques in which signals received by an antenna are processed by addition and subtraction to produce signals in multiple channels which are further processed to determine the range and location of targets. For example, a monopulse system may produce intermediate-frequency sum and difference signals at the antenna processor in response to returns received from the target. The sum and difference signals, together with other signals, are downconverted and digitized for further processing. It is well known that the analog components such as antennas, transmission lines, couplers, filters, and the like may have phase and amplitude differences. The analog processing which produces the sum and difference signals, as well as other signals, may therefore have differential phases or attenuations which result in analog signals which do not correctly represent the signals incident upon the antenna. These errors may be reduced by proper alignment.
In order to perform further processing on the analog intermediate-frequency (IF) sum (.SIGMA.), alpha difference (.alpha..DELTA.), beta difference (.beta..DELTA.), and sidelobe (SL) signals, the signals must be downconverted to baseband. Each of the analog signals is normally downconverted to baseband by a pair of mixers, which receive as local oscillator signals a pair of mutually quadrature sinusoids at the IF frequency, so that one mixer of the pair produces an in-phase (I) signal and the other produces a quadrature (Q) signal. Thus, at least eight mixers are required to downconvert four analog signals to baseband. For further processing, the signals must be in digital form, and as a consequence eight analog-to-digital converters (ADC) are used. Naturally, if there are more than four signals to be processed, additional mixer pairs and ADC's are required. Each ADC performs a sample-and-hold function, followed by quantization and digital conversion.
Correct processing of the sum, .alpha. difference, .beta. difference, and sidelobe signals depends upon knowledge of their relative amplitudes and relative times of arrival. Those skilled in the art know that mixers are subject to amplitude and phase errors when downconverting, due to differences in the capacitance and forward resistances of the diodes used. The filters used in conjunction with a downconverter may have slight differences in alignment which result in group delay (time) differences. Furthermore, analog-to-digital converters are subject to time jitter attributable to the sample-and-hold function, and also suffer from nonlinearity in conversion. These problems are exacerbated by the fact that the time and amplitude errors are often temperature-dependent. This results in a situation in which the I and Q components of each digitized signal may not have amplitudes which correctly represent their relationship to the Q and I components, respectively, of the source signal. Furthermore, the digitized value of a sample may represent the magnitude of the signal being downconverted and digitized at a time which is not precisely correct; as for example a sequence of samples may actually represent the value of the analog signal as if it were sampled at 2.degree. , 89.degree., 181.degree., 268.degree., 2.degree. rather than 0.degree., 90.degree., 180.degree., 270.degree., 0.degree.. These errors in the downconversion of any one analog signal make it difficult to accurately assess the magnitude of the signal at any moment. The problem becomes acute when several such signals must be mutually compared, as with monopulse processing. The errors in processing each separate signal may result in much larger errors in the processing of the signals in combination. For example, the difference between two large numbers may exhibit large percentage errors even if the percentage error of each large signal is small.
For these reasons, it is desirable to reduce the errors occurring in the downconversion and analog-to-digital conversion of multiple channels of signals.